1. Field of the Invention
The present invention relates to antennas and more specifically to an apparatus and system to combine broadband electric and magnetic antennas so as to create a highly efficient electrically small broadband antenna.
2. Description of the Prior Art
Broadband antenna systems are in great demand for precision tracking, radar, and communications. A commercially successful UWB antenna system must be both small and efficient. Additionally, it is advantageous for a UWB antenna to radiate and receive signals with polarization diversity.
In related art, Chu, Kraus, and Schantz have considered the theoretical advantages of an electric-magnetic antenna system in which fields from an electric element are arranged ninety degrees out of phase with respect to fields from a magnetic antenna element, i.e. fields in quadrature. Chu argues that such a composite antenna could be made half the size of a standard small element electric or magnetic antenna [L. J. Chu, “Physical Limitations of Omni-Directional Antennas,” Journal of Applied Physics, 19, 1948, pp. 1163–1175]. Kraus observed that feeding orthogonal loop and dipole elements leads to quadrature signals [John Kraus, Antennas, 2nd ed. New York: McGraw Hill, 1988, p. 264, Problem 6–9]. Also, the inventor has elsewhere observed that there is a beneficial cancellation of near field components around co-located ideal Hertzian electric and magnetic point dipoles [Hans Gregory Schantz, “The Energy Flow and Frequency Spectrum About Electric and Magnetic Dipoles,” Ph.D. Dissertation, The University of Texas at Austin, August 1995, pp. 51–52]. This cancellation results in a fixed, net radial outward energy flow about the antenna. In principle, this should lead to a significantly smaller antenna with less troublesome near field reactive energy than could be achieved by a standard small element electric or magnetic antenna.
In other art, Barnes et al teach a UWB chiral system involving relative delays between signals to or from a pair of orthogonal antennas [U.S. Pat. No. 5,764,696]. This art does not address methods other than a delay for achieving quadrature signals, nor does this art teach how to achieve a substantially omni-direction chiral-polarized transmission or reception.
To achieve a broadband electric-magnetic antenna system requires a superposition of both a broadband electric element and a broadband magnetic element. First, this section will address broadband electric antennas. Second, this section will address broadband magnetic antennas. Finally, this section will examine antenna systems comprising superpositions of electric and magnetic antenna elements.
Broadband Electric Antennas
A wide variety of broadband electric antenna elements have been proposed. This section will survey the most relevant and applicable. Walter Stöhr introduced solid, surface-of-revolution spheroidal and ellipsoidal broadband antenna elements [U.S. Pat. No. 3,364,491]. Farzin Lalezari et al devised a semi-circular dipole or dual notch antenna element [U.S. Pat. No. 4,843,403]. Mike Thomas et al proposed planar cross-sections of spheroidal dipoles or planar circle dipole elements [U.S. Pat. No. 5,319,377]. Taisuke Ihara et al suggested multiple plate semi-circular arc elements [U.S. Pat. No. 5,872,546]. In other art, the present inventor introduced a variety of broadband dipole designs [U.S. Pat. No. 6,845,253] as well as planar elliptical dipole antennas fed from a coplanar taper microstrip balun [U.S. Pat. No. 6,512,488; U.S. Pat. No. 6,642,903].
Broadband Magnetic Antennas
A wide variety of broadband magnetic antennas have been proposed. For instance, Barnes taught a tapered broadband magnetic slot antenna [U.S. Pat. No. 6,091,374; U.S. Pat. No. 6,400,329; U.S. Pat. No. 6,621,462]. Such antennas can achieve broadband performance, but do not yield omni-directional performance. The inventor suggested a planar loop antenna [U.S. Pat. No. 6,593,886], but this planar loop antenna has a dispersive pattern resulting from the relative delays introduced to signals transmitted or received at different angles.
Harmuth suggested using cloverleaf loop antennas to ensure a uniform delay and non-dispersive omni-directional wave front [Henning Harmuth, Antennas and Waveguides for Nonsinusoidal Waves, Orlando, Fla.: Academic Press, 1984, pp. 98–99]. Cloverleaf loop antennas have long been appreciated by antenna designers for their ability to achieve a distributed loop or magnetic dipole type response with uniform phase behavior around the periphery of the loop [John Kraus, Antennas, 2nd ed., New York: McGraw Hill, 1988, pp. 731–732]. Harmuth further taught that additional shielding was necessary to prevent a superposition of signals from a near and a far side of the cloverleaf loop antenna. Harmuth also failed to disclose how to implement a well matched broadband cloverleaf loop antenna with acceptable performance.
Electric-Magnetic Antennas
A wide variety of composite electric-magnetic antennas have been proposed. One early design was the superposition of a dipole antenna along the axis of a loop antenna disclosed by Runge [U.S. Pat. No. 1,892,221]. Runge's polarization diversity receiver allows the detection of a signal with any polarity at a particular frequency, but because the phase difference between the two elements depends upon a quarter wavelength difference in the length of a transmission line, it achieves the desired effect of a 90° phase shift only at a particular frequency.
Luck [U.S. Pat. No. 2,256,619] and Busignies [U.S. Pat. No. 2,282,030] both proposed various superpositions of loop and dipoles antennas. Additionally, Kandoian proposed an “electric-magnetic antenna” that could operate over relatively narrow bandwidths [U.S. Pat. No. 2,465,379]. Kandoian further addressed the performance of his electric-magnetic antenna system elsewhere [Kandoian, “Three New Antenna Types and Their Applications,” Proc. IRE, February 1946, pp. 70W–75W].
Kibler proposed a similar antenna system [U.S. Pat. No. 2,460,260]. Since that time a great many inventors have proposed to superimpose electric and magnetic antenna elements. These superpositions have achieved antenna loading, directionality, polarization diversity, and other goals. None of this prior art addresses the challenging problem of creating an antenna system that can create a quadrature field configuration over a broadband range of frequencies.
In view of the foregoing, there is a need for a compact planar broadband loop antenna that can provide an omni-direction horizontally polarized signal. Similarly, there is a need for a compact, readily manufactured planar electric broadband antenna. There is a further need for smaller, more efficient broadband antennas than are currently available with electric only or magnetic only small element antennas. There is also a need for an antenna with minimal stored reactive energy and thus maximal bandwidth. There is a further need for an antenna with minimal reactive energy and thus minimal undesired coupling with a surrounding environment within which the antenna is embedded.